An ultrasonic motor (USM) is a type of electric motor that causes ultrasonic vibrations in piezoelectric elements in the motor, which in turn induces motion of motor components. Comparing to other types of motors, USMs have lower vibrations. Thus, USMs may be used as actuators in low vibration applications such as camera motors.
USMs may be driven by alternating current (“AC”) signals. Ideally, for efficiency, the driving signals that are applied to a USM would be tuned in frequency, phase and/or amplitude to match operational characteristics of the USM. Driver circuits that generate the driving signals typically are designed to match reference characteristics of a USM, for example, the characteristics identified by USM manufacturers in their data sheets. In practice, however, characteristics of manufactured USMs do not always match their reference characteristics. Characteristics of USMs may vary from the reference characteristics across different manufactured lots due to manufacturing variances. Characteristics of a single USM also may vary over time at the USM ages. Traditional driver circuits oftentimes cannot adjust their driving signals to compensate for these variances.
Thus, there remains a need for a motor drive control system that responds dynamically to changing operating conditions in an ultrasonic motor.